Tree mixtures increase bird taxonomic and functional diversity over pure stands of tree species planted outside their natural range—but not over pure native stands

Recent biodiversity loss has emphasized the necessity to critically
evaluate the consequences of human alterations of forest ecosystems. Stand
diversification via tree species mixtures and the use of non-native tree
species are two such alterations currently gaining importance as climate
change adaptations. However, the effects of local versus regional tree
mixing on associated biodiversity and notably the modifying role of tree
species growing outside their natural range remain poorly understood. We
assessed how monocultures and mixtures of native and introduced tree
species influence the taxonomic and functional diversity of northwest
German bird communities at stand and landscape scales. We focused on the
dominant natural tree species (Fagus sylvatica) and economically important
conifer species planted outside their natural range (the native Picea
abies and non-native Pseudotsuga menziesii). We found that bird species
richness and functional diversity were generally higher in pure and mixed
stands of native F. sylvatica than in pure conifer stands, especially in
comparison to non-native P. menziesii. These differences were particularly
strong at the landscape scale. Pure conifer stands harbored only a reduced
set of functionally similar bird species. Structural diversity based on
tree microhabitat availability emerged as a key predictor of bird
diversity. Synthesis and applications: Our study suggests that tree
species mixtures do not necessarily increase bird diversity compared to
pure stands of native trees, but can promote bird diversity relative to
pure stands of species planted outside their natural range. Moreover,
local mixtures, rather than a mosaic of pure stands, may promote bird
diversity also at the landscape scale. By contrast, pure stands of tree
species planted outside their natural range can increase the biotic
homogenization of forest birds. Promoting structural diversity of
microhabitats via tree retention and ensuring that non-native trees are
planted in mixtures with native trees may alleviate potential limitations
of climate change-oriented management for biodiversity.

当前生物多样性丧失的严峻形势，凸显了批判性评估人类干预森林生态系统所产生后果的必要性。通过营造多树种混交林以及栽植外来树种实现林分多样化，是当前作为气候变化适应策略而愈发受到重视的两类人为森林生态系统改造方式。然而，局域与区域尺度下的林木混交对伴生生物多样性的影响，尤其是生长于自然分布区外的树种所起到的调控作用，目前尚未得到充分阐释。本研究评估了乡土树种与引入树种的纯林及混交林，在林分与景观尺度下对德国西北部鸟类群落的分类多样性与功能多样性的影响。本研究聚焦于德国西北部的优势乡土树种——欧洲山毛榉（Fagus sylvatica），以及栽植于自然分布区外的经济重要性针叶树种：乡土云杉（Picea abies）与外来树种北美黄杉（Pseudotsuga menziesii）。研究结果显示，乡土欧洲山毛榉的纯林与混交林内的鸟类物种丰富度与功能多样性，整体上均高于针叶树纯林，尤其是相较于外来树种北美黄杉纯林时差异更为显著。这类差异在景观尺度上表现得尤为突出。针叶树纯林仅能维持一类功能趋同的鸟类类群，物种丰富度显著降低。基于林木微生境可获得性的结构多样性，成为预测鸟类多样性的关键因子。研究总结与应用启示：本研究表明，相较于乡土树种纯林，多树种混交未必能提升鸟类多样性，但相较于栽植于自然分布区外的树种纯林，混交林可有效促进鸟类多样性。此外，局域尺度的林木混交而非纯林镶嵌格局，同样可在景观尺度上提升鸟类多样性。与之相反，栽植于自然分布区外的树种纯林会加剧森林鸟类的生物均质化现象。通过保留林木以提升微生境结构多样性，并确保外来树种与乡土树种混交栽植，可缓解以气候变化适应为导向的森林管理措施对生物多样性造成的潜在限制。